1. The Field of the Invention
This invention relates to the transmission and reception of high speed digital data, and more particularly combined with a composite TV signal such as that typically transmitted or broadcast in the U.S., Japan, Europe, or any other country utilizing fixed bandwidth channel transmission standards, e.g., NTSC, PAL and SECAM. More particularly, the invention relates to reallocation of a portion of a composite video signal, i.e., the vestigial sideband in a composite video transmission signal, for use as a high speed digital data subchannel.
2. Background of Related Art
The transmission of high speed data, e.g., 5 Mb/s, through a cable TV, wireless or broadcast TV channel is known. For instance, U.S. Pat. No. 5,247,364 discloses the dedicated use of TV channels for in-band data transmissions. U.S. Pat. No. 5,513,180 discloses time-division multiplexing of TV signals with various bit rate digital data sources. However, systems such as these assume unlimited transmission resources, i.e., an unlimited number of available TV channels, and make inefficient utilization of the existing TV channel resources. In many locales, all available TV channels are already utilized. Yet, the need to transmit high speed data through the existing cable TV or broadcast TV facilities persists, particularly with increased consumer demand for digital data services related to the use of the Internet. Moreover, in the U.S., local `must carry` laws for cable companies essentially require cable companies to allocate some of their limited number of TV channels to local broadcasts, thus making fewer channels available for digital data transmission. Accordingly, a need exists to invisibly transmit high speed digital data along with a composite TV signal in the same channel without affecting the visible quality or content of the composite TV signal.
For decades, designers have been constrained by TV transmission standards chosen many decades ago, first with the development of monochrome TV, and then, in the 1950's and 1960's with the development of the transmission standards for color TV. To this end, typical TV broadcast channels are limited in bandwidth, e.g., to 6 MHz in the U.S.
Standards for TV signals utilized throughout the world have already `squeezed` color TV signals into a fixed width channel which was originally designed for only black-and-white TV. For instance, to squeeze a larger bandwidth video signal into a fixed-width TV channel, conventional systems have bandwidth-filtered the lower sideband of the composite TV signal to form what is called a vestigial sideband (VSB). Thus, as is now well known, video modulation used in conventional television transmission apparatus amplitude-modulates (AM) an analog baseband signal to convert it into a double sideband (DSB) modulation signal, and limits the bandwidth of the lower sideband modulation signal using a band-pass filter included with an intermediate frequency circuit or a transmission frequency circuit, thus obtaining the vestigial sideband (VSB) modulation signal. The formation of the vestigial sideband from the larger lower sideband enables a video signal, e.g., an 8 MHz wide signal, to be transmitted in a smaller, fixed-bandwidth transmission channel, e.g., a 6 MHz bandwidth channel in the U.S.
In addition to bandwidth limiting an analog signal to obtain the VSB, it is also known to form a digital type vestigial sideband from a digital baseband signal. For instance, U.S. Pat. No. 5,327,462 discloses the formation of a vestigial sideband (VSB) modulation signal from a digital baseband signal.
Originally, the VSB modulation signal was formed using analog filters formed with inductors, capacitors and resistors. However, the edges of the filter were far from the ideal square wave and thus a fair amount of `slop` was associated with the VSB, causing interference with the adjacent TV channel.
More recently, Surface Acoustic Wave (SAW) filter technology has been utilized to form the VSB because it exhibits very sharp edges (i.e., are closer to a square band-pass filter) as compared with filters formed from inductors, capacitors and resistors, at much lower cost. Thus, interference between adjacent channels was better avoided, but the full bandwidth of the TV channel, e.g., 6 MHz in the U.S., was still required to transmit a conventional composite signal over a TV channel.
TV channels have been `squeezed` even further by a more recent implementation of stereo audio, i.e., left and right audio channels, as well as a secondary audio program (SAP)channel and a professional audio channel. As with the implementation of color TV, the stereo audio was implemented with backward compatibility, i.e., without rendering current TVs obsolete.
Other conventional systems utilize time-division multiplexing to insert digital data into the vertical blanking interval of a composite TV signal. The vertical blanking interval allows the intermittent placement of digital data in the composite TV signal. For instance, close-captioning or teletex systems utilize the vertical blanking interval of a composite TV signal. However, if the vertical blanking interval is already in use for providing close-captioning or other text services, then the channel cannot carry additional digital data. Moreover, this technique requires the cable or broadcast TV operator to have advance knowledge of whether or not the TV signal being carried in a particular channel is already utilizing the vertical blanking interval.
U.S. Pat. No. 5,063,446 discloses an apparatus to combine a digital audio signal with a standard TV signal. According to this patent, an auxiliary carrier is added to the video signal centered about 0.75 MHz below the video carrier. However, as shown in FIG. 1 of this patent, the auxiliary spectrum of the auxiliary carrier, while conditioned to occur between the spectral bands occupied by the luminance component, nevertheless affects to a certain extent the content of the composite TV signal. Moreover, while this technique is disclosed as being applicable to inserting an auxiliary audio channel having a frequency band of only a few tens of KHz, digital data in the several Mb/s range would likely obliterate the composite TV signal to too great an extent.
U.S. Pat. No. 4,907,218 proposes a system which superimposes a second carrier onto the first which is identical in frequency to the first carrier but different in phase by 90 degrees. However, as disclosed in the patent, such systems impair the composite TV.
Thus, the need exists to transparently transmit digital data at a high speed, e.g., 5 Mb/s, together with a conventional composite TV signal, in a single standardized TV channel, without affecting the quality or content of the composite TV signal.